FIG. 1 illustrates a typical Raman microspectrometer 10. The Raman microspectrometer 10 includes an optical microscope 20, coupled via supports 32 and 34 to a combined excitation laser source/spectrometer 30. The Raman microspectrometer is used to analyze the molecular structure of a sample that is disposed on the microscope stage 22. During analysis the sample is secured to the stage 22 and laser beam pulses are directed via the optical transfer tube 33 through the lens of the microscope 20 onto points in the sample. Resulting Raman and Rayleigh scatter from the sample is forwarded back through the microscope lens and optical transfer tube 33 to the spectrometer. The spectrometer filters out the Rayleigh scattered energy and separates the wavelengths of the Raman scattered energy to identify the molecular structure at examined points of the sample.
The stage 22 on which the sample is disposed is motor controlled by the joystick 15 to provide movement (i.e., travel) of the stage along the x, y and z axis to thereby allow analysis of each point in the sample. In general, the size of the stage is designed to accommodate slides and/or semiconductors or other types of samples for which Raman Microspectroscopy has been shown to be appropriate. For example, the stage of the microspectrometer in FIG. 1 has a four inch by four inch x/y travel capability, which is generally sufficient to examine any sample that fits within the stage.
However it is sometimes desirable to perform Raman analysis on samples having a size that exceeds that of an existing optical microscope stage. An example of such a sample is a digital mammography panel that is used in x-ray imaging systems, also referred to as a flat panel detector. Flat panel detectors may be comprised of a thin film transistor layer coated with one or more material layers including a photoconductive layer such as amorphous selenium. Exemplary layers of a flat panel detector 50 are shown in FIG. 2 to include a top electrode 52, a charge barrier layer 53 (typically made of Parylene-N) separating the top electrode from an amorphous selenium-based charge generator layer 54, and a charge collection electrode layer 55 disposed upon a thin-film transistor (“TFT”) array 56.
Under normal operation, before exposure to x-ray radiation, the photoconductive layer is uniformly biased relative to electrical charge readout means by application of a biasing field via voltage source 58. As x-rays are directed at the panel, electrons move from the valence band to the conduction band thereby creating holes where electrons once resided. Electron-hole pair charges move in opposite directions along electric field lines towards opposing surfaces of the photoconductive layer. Holes collected by the electrode 55 are used to charge capacitors in the TFT array 56 which may subsequently be read out to provide a latent image.
The accuracy of image capture is thus highly dependent upon the ability of the electron hole pairs to travel freely within the photoconductive layer. However anomalies in the manufacturing process may give rise to defects within the amorphous selenium that impair the free movement of electron hole pairs. For example, temperature changes or other processing procedures may cause crystals to be generated in the selenium. Before the panel may be released for commercial use, it is necessary to perform a series of tests on the panel to ensure that the panel is free from such anomalies.
Panel testing may identify spatial coordinates of one or more problems in the panel. A Raman microspectrometer is preferably used to determine the molecular structure at the coordinate of interest. However it is difficult to use existing Raman microspectrometers to analyze digital image panels in their entirety because the size of the flat panel cannot be accommodated by the existing stage and travel capabilities of the microspectrometer. Digital mammography panels may measure more than eleven by nine inches, while the travel distance of available microspectrometer stages are only ( ) four inches or less in each dimension. In addition, even if the travel of the existing stage could be adjusted, the physical space constraints between the microscope 20, optical transfer tube 33, and spectrometer 30 limit the ability to properly examine the entire panel.
As a result, inspection of problem coordinates of a mammography panel requires destruction of the panel. Panels are cut into discrete sections that can be examined using the current stage travel capabilities. After destruction, a technician would iteratively step through each pixel position of each panel section to locate and analyze anomalies caused by the manufacturing processes. This process was time consuming, destructive and concomitantly expensive. It would be desirable to identify a non-destructive apparatus and method for analyzing oversized samples using micro spectrometers.